1. Field
The disclosure relates to communications circuitry, and in particular, to a receiver front end supporting carrier aggregation.
2. Background
Receiver (RX) carrier aggregation is a feature supported in modern wireless devices wherein signals on multiple carriers are simultaneously processed by a receiver. For example, a low-noise amplifier (LNA) of a receiver may simultaneously amplify a first signal corresponding to a first carrier and a second signal corresponding to a second carrier, and supply the amplified first and second signals to first and second loads, respectively, for further processing.
A receiver supporting carrier aggregation (CA) is typically also required to support non-carrier aggregation (non-CA) operation for backwards compatibility. In such cases, the performance of the LNA should be maintained across both the CA and non-CA modes. For example, it would be desirable to design the LNA such that the gain provided to the received signal is not significantly affected by whether the receiver is operating in CA mode or non-CA mode, a feature also denoted herein as “gain alignment” across modes. Gain alignment helps the receiver meet sensitivity requirements for both modes, and also simplifies the baseband settings and modem programming requirements. It would further be desirable to maintain the input matching properties of the LNA across CA and non-CA modes for optimum performance.
Certain prior art receivers implement CA mode by providing separate input transistors, e.g., by providing a separate cascode amplifier for each carrier signal. A drawback of this approach is that more than one external matching component may be required to maintain matching across modes. For example, according to this approach, an L-C (inductor-capacitor) match or L-L (inductor-inductor) match may be needed to preserve the matching across non-CA and CA modes. Other prior art implementations provide a single input transistor having a drain coupled to multiple cascode transistors (e.g., one cascode transistor for each carrier). A drawback of such an approach is that the quality of the matching will generally degrade in CA mode versus non-CA mode, due to the different levels of current required for the modes. Furthermore, more power may be needed in CA mode to maintain gain alignment.
Accordingly, it would be desirable to provide a receiver front end that efficiently maintains gain alignment across both CA and non-CA modes while minimizing the number of required external matching elements.